Oh helper throughout the ages.
Oh glory of the apostles.
Oh shining light to those who dwell in Galicia.
Oh you who aid the pilgrims.
James, remover of hardships,
take off the chains of our sins and lead us to safe harbors.

You who come to the aid of those who call to you in their perils
both at sea and on land,
help us now and when we face the danger of death.
And lead us to the door of salvation.

Grant us a safe harbor
On the day of Judgement

So that with God
Who had no beginning

And with his son
Who is without end

And with the Paraclete
Issuance of both

Saved from the black
Cavern of Hell

And admitted into the
holy choir of angels

Purged of all sin,
Filled with joy
With the reward of life

And, with you as our leader and protector
We may enter with pious sing
Into the garden of Paradise.

The YouTube page says that this is "medieval chant from Codex Calixtinus," and offers also the following information:

If, though, the folio number there is right, this comes from Matins and not the Mass; I've also seen it listed elsewhere as a "Responsory." It's from Book I, the "Book of Liturgies." To me, the interesting part here is the section that begins "Portum in ultimo"; it seems to play the part of a doxology, but sort of acts like trope material too. I'm not sure where it comes from, since it's not included in other recordings of this material - but it is a beautiful text. The content itself seems a bit over-the-top even for the celebration of an Apostle's feast day; perhaps the fact that this was a pilgrimage/festival (St. James was celebrated, apparently, twice a year: July 25 and December 30) explains that.